We report the observation of time resolved self-defocusing effects in metal cluster compound (n-Bu4N)4[Mo8Cu12O8S 24]. We also present the dispersion of the nonlinear refractive index, n2, of the compound measured over a spectral range below the resonance 2.45 eV. The time resolved self-defocusing effects clearly indicate that the observed nonlinear refraction should be of electronic origin. The dispersion can be fit by a two-level model. Away from the resonance, n2 = -2 × 10-11 cm2 W-1 M-1, while at 532 nm, just below the resonance, n2 = -(2.3 ± 0.5) × 10-9 cm2 W-1 M-1. In addition, the measurements reveal the involvement of excited state absorption. Based on these observed nonlinearities, optical limiting of the compound has been demonstrated with nanosecond laser pulses of 532 nm wavelength.